These days elevation of the contrast ratio (CR) in liquid-crystal display devices is being promoted. In particular, a VA-mode liquid-crystal display device has the advantage that CR in the normal direction (hereinafter referred to as “front CR”, and “front CR” may be generally called also as “on-axis contrast ratio”) is high as compared with that in other modes, and various studies and developments are now made for further enhancing the advantage. As a result, in these 6 years, the front CR in VA-mode liquid-crystal display devices has increased from about 400 to about 8000, or by about 20 times.
For example, as one means of increasing transmittance, there is known a color filter-on-array (COA) structure (for example, JP-A 2005-99499, 2005-258004, 2005-3733). The COA structure may have an increased aperture ratio and therefore may increase the transmittance in the black or white state of display. At present, the interest in environmental problems is increasing, and employing the COA structure to increase the transmittance contributes toward reduction in power consumption and is favorable from the viewpoint of the environment.
The front CR is determined by two transmittance factors in the while state of display and in the black state of display (white brightness and black brightness), and therefore could not be elevated by mere increase in the transmittance. Even though the transmittance in the white state of display could be increased but when the transmittance in the black state of display is also increased simultaneously, then CR elevation could not be achieved. In order to increase the front CR by employing a structure capable of improving the transmittance in the white state of display, it is important to prevent the increase in the transmittance in the black state of display by the employment of the structure.
On the other hand, in liquid-crystal display devices, it is important that not only the front CR is high but also CR in oblique directions (hereinafter this may be referred to as “viewing angle CR”) is high. Various techniques of using one or more retardation films have been proposed for reducing the light leakage in oblique directions in the black state of display in VA-mode liquid-crystal display devices (for example, JP-A 2006-184640). In general, a retardation film is disposed on both the front side and the rear side of the liquid-crystal cell existing in the center therebetween, in which retardation necessary for optical compensation in the display device is distributed into the two retardation films. In general, two systems are employed for the combination for optical compensation. In one system, the retardation films each separately disposed on the front side and on the rear side equally have same retardation; and the advantage of the system is that the films of the same type can be used therein. In the other system, the retardation film disposed on either one side is made to have larger retardation; and the system is advantageous in point of the cost since it enables optical compensation by the use of a combination of inexpensive retardation films. In the latter system, in general, the retardation film to be disposed on the rear side is made to have larger retardation in practical use. One reason is the production cost. Regarding this reason, JP-A 2006-241293, paragraph [0265] says as follows: “In case where the cellulose acylate film of the invention is used only as the protective film, disposed between the liquid-crystal cell and the polarizing film, of one polarizing plate, this may be on either side of the upper polarizing plate (viewers' side) or the lower polarizing plate (backlight side) with no functional problem. However, when it is used on the side of the upper polarizing plate, the functional film must be provided on the viewers' side (upper side) and the producibility may be thereby lowered, and therefore, it may be used on the side of the lower polarizing plate in many cases, and this may be a more preferred embodiment.” The second reason is that disposing the film having larger retardation on the rear side is preferred from the viewpoint of the impact resistance and the resistance to environmental change including temperature change and humidity change.